Protein kinase C (PKC) serves important functions in signal transduction. We hypothesized that PKC modulation of microvascular permeability to macromolecules is mediated by nitric oxide (NO). To test this hypothesis, we stimulated PKC topically with 10(-7) M phorbol 12,13-dibutyrate (PDBu) in the hamster check pouch microcirculation. NG-monomethyl-L-arginine (L-NMMA) at 10(-4) M was superfused in a bicarbonate buffer solution throughout the experiment to inhibit the activity of NO synthase. We evaluated changes in transport of fluorescein isothiocyanate-labeled 150,000 mol wt dextran by integrated optical intensity (IOI) using intravital fluorometry and computer-assisted digital image analysis. Postcapillary areas were recorded. PDBu increased IOI from baseline to a value of 46.8 +/- 6.3 units (+/- SE). Pretreatment with L-NMMA decreased the PDBu-stimulated increment to 10.8 +/- 0.9 units. These results demonstrate that PKC-activated modulation of macromolecular transport operates through a mechanism involving the production of NO.